As far as I understand, the sloppy ones are the sysadmin behind those sites, not the WordPress installs. And as far as I know, lazy sysadmins don't discriminate regarding what technology they are lazy with.
Software which is easy to set up initially, but hard to upgrade, tends to be much more likely to be out of date than software which is easier to upgrade.
Things that consist of "a big blob of PHP plus a lot of extensions" tend to fall into this camp. PHP apps are generally very easy to get set up the first time, and popular ones like Wordpress have lots of extensions you can add on; but then once you've been running that for a while, you discover that if you upgrade, these n extensions all break, and rather than bothering to find replacements, you just don't bother upgrading.
How do you fix this? Make upgrading easy, don't rely on extensions, or make sure you have bulletproof, stable APIs so that no one worries "if I upgrade, all these things are going to break."
